Картонные коробки поместятся в грузовой отсек, разве что одна большая коробка.

Картонные коробки is the subject of the sentence, so it is in the nominative plural.

• коробки = boxes
• картонные = cardboard, agreeing with коробки in gender/number/case

Russian usually uses an adjective here where English uses a noun as a modifier:

• cardboard boxes → картонные коробки

So this is the normal way to say cardboard boxes in Russian.

Поместятся is the 3rd person plural future form of поместиться.

A key point is that поместиться is a perfective verb. Perfective verbs do not have a true present tense in Russian; their “present-looking” forms actually refer to the future.

So:

• поместятся = they will fit
• more literally: they will manage to fit / they will be accommodated

In English, depending on context, this can also sound like should fit or can fit, but grammatically in Russian it is future.

Поместиться means to fit, to be accommodated, or to have enough room to go inside something.

In this sentence, it means that the boxes will physically fit inside the cargo compartment.

Compare:

• поместить = to place / to put something somewhere
• поместиться = to fit / to be able to go in

So:

• Мы поместим коробки в отсек = We will place the boxes in the compartment
• Коробки поместятся в отсек = The boxes will fit in the compartment

The -ся ending often gets called reflexive, but in many verbs it is simply part of the verb’s meaning and is not literally translated as oneself.

Here, поместиться does not mean something like place themselves in a literal English sense. Instead, it is just the normal verb meaning to fit / to be accommodated.

So it is best to learn поместиться as a whole vocabulary item.

Because Russian uses:

• в + accusative for motion into something, or a change/result of ending up inside
• в + prepositional for location inside something

Here the idea is that the boxes will fit into the cargo compartment, so Russian uses в грузовой отсек.

Compare:

• Коробки поместятся в грузовой отсек. = The boxes will fit into the cargo compartment.
• Коробки уже в грузовом отсеке. = The boxes are already in the cargo compartment.

So:

• отсек = accusative
• отсеке = prepositional

Here разве что means something like:

• except perhaps
• maybe except
• unless it’s
• with the possible exception of

It introduces a small exception or reservation.

So the sentence means roughly:

The cardboard boxes will fit in the cargo compartment, except perhaps one large box.

This phrase often softens what is being said: the speaker thinks the statement is true, but adds one possible exception.

This is an elliptical structure: Russian often leaves out words that are easy to understand from context.

The full version would be something like:

Картонные коробки поместятся в грузовой отсек, разве что одна большая коробка не поместится.

So after разве что, the speaker omits не поместится because it is obvious.

Одна большая коробка is in the nominative because it is the subject of that implied clause:

• одна большая коробка (не поместится)

This kind of omission is very natural in Russian.

Because the speaker is talking about all the boxes as a group, and then adding one possible exception.

So the logic is:

• the cardboard boxes will fit
• except maybe one large box

The singular is used because only one box is being singled out from the larger group.

Not exactly.

Грузовой отсек means cargo compartment, cargo hold, or storage compartment for cargo. It sounds more general and somewhat more formal/technical.

Багажник is usually:

• a car trunk / boot
• sometimes a luggage rack or baggage area, depending on context

So грузовой отсек fits better if the context is a vehicle, aircraft, or some larger transport space meant for cargo.

A more explicit version would be:

Картонные коробки поместятся в грузовой отсек, разве что одна большая коробка не поместится.

Another natural rewording is:

Все картонные коробки поместятся в грузовой отсек, кроме, возможно, одной большой коробки.

These versions may be easier for a learner to parse, but the original sentence is natural and concise.